1. Field of the Invention
The present invention relates to a system for controlling reception sensitivity in a radio receiver. It particularly relates to a reception sensitivity control system in a sweeping receiver capable of performing preset channel selection.
2. Background of the Invention
A sweeping receiver capable of performing preset channel selection typically includes a receiving station memory system. There has been known such a memory system in which a reception signal is detected by use of a reception station search function such as scanning, seeking, or the like. The reception signal frequency is swept in advance. Upon detection of the reception signal, the sweeping is temporarily stopped and a frequency signal corresponding to the reception frequency at that time is stored in a memory. Thereafter, sweeping is restarted so that reception stations are successively preset.
Japanese Patent Unexamined Publication No. 59-159019 disclosed such a sweeping receiver in which reception sensitivity is lowered in a period of sweeping so as to prevent sweeping from being erroneously stopped owing to noise or the like at a position where no station is located. When no station can be detected in a period of sweeping, the reception sensitivity is increased and the frequency sweeping is repeated. When a sweeping receiver in which reception sensitivity is controlled by such a conventional reception sensitivity control system is additionally provided with a preset channel selection function by means of the above mentioned receiving station memory system, there has been such a disadvantage that the number of preset receiving stations may be so small in comparison with the storage capacity of the memory that there occurs an address in which no receiving station is stored. Therefore, channel selection cannot be attained even if a channel selection switch is operated.